1) Field of the Invention
The present invention relates to a synchronous clock phase control circuit which controls a clock signal to conduct printing control in a color copier, a laser beam printer, or the like.
2) Description of the Related Art
In printing control in a copier, a laser printer or the like, a clock signal synchronized with a trigger signal is used to control a print region on a sheet for each line. FIG. 5 is an input/output signal time chart for a synchronous clock generator which generates a clock signal synchronized with a trigger signal using an input clock. In FIG. 5, the trigger signal is a low active signal. When the trigger signal is of low logical level (“Low”), the trigger signal becomes effective (asserted) to stop the output of a synchronous signal. When the trigger signal is changed (negated) from “Low” to high logical level (“High”), a synchronous signal synchronized with the trigger signal is output after the passage of predetermined synchronous clock start time from a point of this change. Using this synchronous signal, line control over the print region on the sheet is conducted.
Since a monochrome copier or a monochrome printer prints only one color, the print region on the sheet can be controlled using one synchronous clock signal. However, a color copier or a color laser beam printer requires a total of four synchronous clock signals for Y (yellow), M (magenta), C (cyan), and K (black). According to the printing technique used in the color copier, the color laser beam printer or the like, various colors are realized by the subtractive color mixture of three primary colors of Y, M and C. In principle, all colors can be realized from the three primary colors. However, if a black color is realized by the mixture of the three colors, resultant black becomes somber. Therefore, four colors of three colors of Y, M and C and a color K are used in many cases and these colors are printed in the order of Y, M, C, and K. In addition, since the color copier or the color laser beam printer requires a high degree of image processing, it is necessary that the four synchronous clock signals have different delays (phases) from one synchronous clock in order to control four-color printing. Specifically, if a Y control clock having a cycle T is set as a reference, it is necessary that an M control clock has a delay of T/n, a C control clock has a delay of 2T/n, a K control clock has a delay of 3T/n each having a cycle T. Further, these four clocks should be synchronous clocks synchronized with a trigger signal.
However, it is difficult to obtain four synchronous clock signals having different phases from a single trigger signal asynchronous with an input clock. To do so, conventionally, a trigger signal is set for each color or the fine adjustments of clock delays are conducted using an exterior element. Therefore, a clock control circuit becomes disadvantageously complicated and the number of elements disadvantageously increases.